It is often desired to process broadband signals, (e.g., radar signals) which interacted with an item in order to determine the range, velocity, and acceleration of the item. But unfortunately, these broadband signals frequently contain a Doppler shift.
The Doppler effect is typically defined as the apparent change in the frequency of a wave, as a light wave or a sound wave, resulting from relative motion of the source and the receiver. The Doppler shift is typically defined as the magnitude of the change in frequency or wavelength of waves caused by the Doppler-effect.
For a broadband signal, each frequency component is Doppler shifted by the same ratio. The result is a different frequency shift for each frequency component of the signal. For example, a 10% Doppler shift would shift a 30 Hz signal component by 3 Hz while shifting a 300 Hz signal component by 30 Hz. The difference in the absolute frequency shifts complicates the signal processing of broadband Doppler-shifted signals. By performing a Fourier analysis on the signal, it can be shown that the Doppler effect occurs in the time domain as well.
The Doppler effect appears everywhere broadband signals appear. The Doppler effect is particularly evident in acoustic signals because the frequency components of an acoustic signal can span several octaves, or even several decades.
Acoustic signals tend to have low propagation velocities. A correlation may exist between two signals that have different propagation velocities. If a correlation exists, the time that it takes to correlate the signals may indicate the range of the item. Processing becomes much more difficult if the item is moving. If a correction is not made to the Doppler shift, it may be impossible to find a correlation between signals that are related.
Two architectures have emerged in schemes that correct for Doppler shift in signals. These two architectures are the space-integrating architecture and the time-integrating architecture. The features to note in the space-integrating architecture are 1) the integration time is equal to the time aperture of the delay line and 2) one of the signals must be time-inverted in order to achieve correlation. These statements can be shown to be correct by any college level text book on the subject. A mathematical proof here would only obscure the significance of the present invention. The features to note in the time-integrating architecture are 1) it is not necessary to time invert one of the signals and 2) integration time is determined by the photodetector.
Therefore, the space-integrating architecture is appropriate when the duration of the signal(s) to be correlated is/are equal to the time aperture of the delay line and it is convenient to time invert one of the signals. The time-integration architecture is appropriate when the time duration of the signal(s) is/are much longer than the time aperture of the delay line (but less than or equal to the integration time of the photodetector) or it is inconvenient to time-invert one of the signals.
The space-integrating architecture is more suited for radar application where the time-integrating architecture is more suited for processing signals received from two different sites. The present invention proposes new time-integrating schemes for correcting Doppler-shifted broadband signals.
The following U.S. patents where found that deal with time-integrating acousto-optic devices and/or ways to improve the broadband performance of acousto-optic devices: U.S. Pat. Nos. 4,326,778; 4,421,388; 4,426,134; 4,531,195; 4,566,760; 4,722,596; 5,121,248; and 5,153,597. But none of these patents disclose the use of a conical lens telescope to improve the broadband performance of time-integrating acousto-optic devices as the present invention does.